In a simulcast paging system, a central paging terminal typically sends a signal to several transmitters for retransmission into overlapping reception zones. To avoid phase interference in pagers receiving transmissions from multiple transmitters, the signals from all of the transmission sites should be synchronized to within .+-. one microsecond (.mu.s). However, the signal from the central paging terminal often reaches the transmission sites at different times, because it propagates over radio frequency (RF) and/or telephone links of substantially different length. It is thus important to provide an appropriate equalizing delay before the signal is transmitted to a pager from each site to compensate for the different time delays incurred as the signal travels to the transmitters in order to synchronize transmission of the signal from all of the transmitters. The transmitter associated with the link having the longest propagation time does not require any additional equalizing delay to be added before transmitting the signal, but all other transmission sites receiving the signal earlier in time do. The equalizing time delays required to synchronize transmission of the signal from the various sites may range from a few .mu.s to several hundred .mu.s.
Methods for delaying a signal are well known in the art. For example, an analog voice signal that is converted to a digital signal by being sampled at a sample rate, f.sub.s, can be stored in a microcomputer buffer for a desired delay time, and then converted back to an analog signal to achieve a desired equalization delay. However, to achieve a delay of 100 ms, with a resolution of 5 .mu.s, an analog signal sampled to a digital resolution of 13 bits would require a sampling rate of 200 KHz. While 5 .mu.s delay resolution has been acceptable for data rates of up to 2400 baud in the past, a lower value of delay resolution is required for simulcast systems operating at substantially higher data rates. Such straight forward techniques to obtain the required delay resolution for higher baud rates become impractical due to the size of the memory buffer and sampling rate that would be required.
Since the need for equalizing time delays so as to synchronize transmitters in a paging system is well known in the art, a variety of apparatus to provide the compensatory delay in the time before a signal is transmitted in a paging system have previously been developed. For example, as described in U.S. Pat. No. 4,317,220, a fixed delay line is inserted into the input line of each slave (remote transmission site) station to provide the requisite time delay to synchronize the transmitter at that site with other transmitters at different locations. However, details of the delay line are not disclosed. This technique is simply one implementation of the method initially discussed and is unusable at the delay resolution and data rates required for the reasons already noted.
As a further example of prior art solutions to this problem, U.S. Pat. No. 4,255,814 discloses a simulcast transmission system having a control center that includes an adjustable audio delay used to delay the audio or information signal that is broadcast by a simulcast transmitter. The adjustable audio delay disclosed in this reference comprises an integrated circuit bucket brigade device that provides a delay for the signal transmitted in accordance with delay data that are recalled from a storage circuit. The stored delay data for each transmission site is adjustable to compensate for changes in the link between the control center and the transmission site, by reprogramming the adjustable audio delay, thereby maintaining phase coherency at the point where transmissions from more than one site are received. A variable clock frequency input signal is used to determine the delay time provided by the bucket brigade device comprising the adjustable audio delay circuit. The technique for controlling the delay disclosed in this reference lacks the resolution to truly synchronize the transmission sites; moreover, it requires relatively complex circuitry, even though it uses an integrated circuit (IC) specifically designed to provide an adjustable delay. This reference also suggests that the signal to be transmitted can be digitized and a microprocessor used to delay the signal, but the patent does not disclose how a signal is delayed with the microprocessor. Bucket brigade devices are not common circuit components and tend to be expensive. Moreover, these devices tend to introduce an undesirable level of distortion in the signal being delayed. Consequently, this technique for delaying a signal is not an acceptable solution to the problem.
Conventionally, a signal that is to be delayed with a microprocessor is digitized and the digital values representing the signal are stored in a first-in, first-out (FIFO) memory circuit that delays the signal by the time required to fill the storage device. A similar, although more expensive, storage device having a variable buffer capacity can be controlled by a microprocessor or by a coder/decoder (CODEC) so that the capacity of the buffer is changed as necessary to provide a desired delay between the time at which a digital signal is applied to the input of the buffer and the time that it is available at the output of the buffer. As a further alternative, a continuously variable slope technique can be used to provide a programmable delay of a signal. A relatively simple circuit will provide delays from about 0 to 5 ms, in minimum steps of approximately 5 .mu.s; however, this technique causes excessive distortion in the signal.
From the preceding description of the prior art, it should be apparent that an efficient, low cost method and apparatus are required for providing time delays to appropriately equalize the time at which a signal is transmitted by different transmission sites in a paging system. The apparatus and method should be capable of providing delays ranging from a few .mu.s as to about 100 ms, with a resolution in the .+-.1 .mu.s range.